Metal Organic Chemical Vapor Deposition (MOCVD) is a viable process for producing second generation high temperature superconducting (HTS) tapes. MOCVD processes have been used to produce YBCO tapes up to 18 meters long with an end-to-end critical current performance of 111 A. MOCVD offers several advantages over competing processes used to produce HTS tapes.
The main advantage offered by MOCVD is that of high throughput. MOCVD processes have the advantage of a high deposition rate (about 1 micron/minute) coupled with deposition zones that are flexibility in their length. Since the deposition zone in a MOCVD process can be as long and as wide as the coating delivery means, typically a showerhead, the deposition zone is, in essence, unlimited.
This combination of high deposition rate and long deposition zone leads to high throughput. Other advantages of MOCVD include preparation of precursors outside the deposition chamber, which allows continuous provision of precursors very easy for long production runs, non line-of-sight coating which can enable double-sided deposition, absence of high-vacuum pumping, and no target fabrication expense.
These benefits and the increase in interest in producing oxide superconducting, ferroelectric, and dielectric materials are the reason MOCVD technology has demonstrated significant growth in the past two decades. These oxide compounds, with the exception of simple dielectrics such as SiO2 and Ta2O5, tend to be complex in composition and structure. They often involve metallic elements with wide-ranging size, electronegativity, and oxidation states that necessarily require complex organic ligands to form volatile, yet thermally stable, precursor compounds. Consequently, the method of precursor delivery, as well as the type of deposition reactions, may vary drastically between components for any multicomponent oxide.
The characteristics of oxide films deposited with MOCVD processes are highly dependant on the selection of precursor compounds, deposition temperature, deposition environment, and kinetic factors such as the precursor partial pressure and flow rate.
Metalorganic compounds suitable as precursors for most main-group and transition-metal elements are now readily available from commercial sources. There are four categories of metalorganic compounds frequently used: β-diketonates, such as 2,2,6,6-tetramethyl-3,5-heptanedionate (thd) and 2,4-pentanedionate (also known as acetylacetonate, acac); alkoxides, such as ethoxide, isopropoxide, and butoxide; alkylmetals, such as ethylzinc and phenylbismuth; and carboxylates, such as benzoate and ethylhexanoate (eha).
β-diketonates are readily prepared by directly refluxing the respective β-diketones with lower alkoxides of the metal element in appropriate solvent. One detriment associated with most common β-diketonate precursors is the need to perform the depositions at reduced pressure, typically ranging from about 1 torr to 25 torr, due to the low volatility of the precursor. Thus, it would be desirable to utilize a precursor with a volatility sufficiently high to permit high pressure or atmospheric pressure vapor deposition.
The vapor pressure of precursor compounds at various temperatures has far-reaching consequences for the quality of the deposited coating. A key aspect in the use of multicomponent precursors in the deposition of oxide coatings is the control of the composition of the precursors in the vapor phase. Since the majority of the oxide systems of present interest have melting points much higher than the typical deposition temperature, insufficient diffusion results in nonequilibrium depositions. Consequently, the composition of the deposited films is highly dependent on the individual precursor partial pressure immediately above the substrate surface.
Liquid precursor delivery systems are commercially available for MOCVD applications. By dissolving the precursors in appropriate solvents and at appropriate concentration, metalorganic precursors with low volatility and poor thermal stability can be accommodated. Such a liquid-injection method is similar to spray pyrolysis in that the composition of the vapor-phase species can be prescribed to a certain degree. The general applicability of such liquid-delivery evaporation systems has been demonstrated for oxide systems such as BaTiO3, YBa2Cu3O7-d, YSZ, LaSrCoO3, and copper using mixtures of respective precursors in the solution.
It should be recognized that, between the preferred characteristics of high volatility and high thermal stability, difficulty remains in identifying appropriate metalorganic compounds as precursors. Liquid injection and flash evaporators are important advances because a wider variety of metalorganic compounds can now be used in the MOCVD of oxides. However, those techniques are limited to precursors that have sufficient solubility in appropriate solvents. In addition, when liquid is directly injected on a substrate surface, the uniformity and the density of the layer, and ultimately the compositional and structural perfection of the deposited layer, have been hard to control.
The precursor compound must deliver metallic elements to the substrate deposition zone at a precisely controlled, constant dosage. The precursor should play an active role in regulating the relative ratio between metal elements, and the single-precursor approach is an important step in that direction.
Finally, the precursor compound should permit at least one reaction mechanism through which a clean deposition reaction with little contamination can be carried out.
A significant drawback of MOCVD to the utilization of the commonly used tetramethyl heptanedionate (thd) precursors is the cost; typically the cost of the thd precursors is about $ 10 to $ 15/g for quantities of 100 g, which is sufficient to produce about ten meters of coated substrate tape. With increased production volume per coating run from the current 10 m to the expected few thousand kilometers, the cost is expected to be reduced by an order of magnitude. The order-of-magnitude reduction in precursor cost is expected to be sufficient to reach a cost goal of $ 10/kA-m. Even so, the precursor cost will be a significant contributor to the overall cost of the HTS material.
Therefore, it would be beneficial if precursors other than the typical thd precursors could be used to produce high quality second generation HTS tapes. If a chemical vapor deposition process and apparatus were developed using such alternate precursors, then a) the cost goal of $ 10/kA-m could potentially be reached with a smaller production volume, in which case widespread use of second generation tapes can be achieved at an earlier time frame and b) reliance on thd-based precursor manufacturers to reduce the cost would not be necessary.
It is thus an object of this invention to provide a chemical vapor deposition based on precursors alternate to thd.
These objects and other features, aspects, and advantages of the present disclosed embodiments will become better understood when the following detailed description is read with reference to the accompanying drawings.